Microspectrophotometry with the DNA-localizing fluorochrome DAPI was used to quantify nuclear DNA content in eight species representing three genera of the subaerial green algal order Trentepohliales (Chlorophyta). Comparisons of mean fluorescence intensity (If) values of algal nuclear genomes to those of chicken erythrocytes (RBC) resulted in an estimate of 1.1-4.1 pg for the algae. DNA levels in Cephaleuros parasiticus Karsten for 2C nuclei in gametophytic phase closely approximate 50% of the 4C values in the sporophytic phase, confirming previous observations based on culture and ultrastructural studies on the presumptive sexual life cycle in this genus. The alternation of generations in this species was not indicated in the original description by Karsten or in any subsequent studies. Genome quantification data for eight trentepohlialean taxa suggest a doubling sequence for nuclear DNA contents. This phenomenon might reflect the presence of a polyploid series in the order Trentepohliales

Key words: C-value > >genome quantification, C-value, Cephaleuros, Chlorophyta, DAPI, nuclear genome quantification, Trentepohliales